A butterfly valve, which is arranged upstream of an intake manifold and adjusts the flow rate of the air which is fed to the cylinders, may be included in internal combustion engines. A typical currently marketed butterfly valve has a valve body provided with a tubular feeding pipe through which the air aspirated by the internal combustion engine flows; a butterfly plate, which is keyed onto a rotating shaft to rotate between an opening position and a closing position of the feeding pipe, is accommodated inside the feeding pipe. The rotation of the butterfly plate is controlled by an actuator device normally comprising an electric motor coupled to the rotational butterfly plate shaft by means of a gear transmission and at least one spring which pushes the butterfly plate shaft to the closing position.
A position sensor, which is adapted to detect the angular position of the rotational shaft (i.e. of the butterfly plate) is coupled to the rotational shaft carrying the butterfly plate; in modern butterfly valves, the position sensor is of the contactless type, i.e. comprises a rotor integral with the rotational shaft and a stator, which is arranged in fixed position, facing the rotor and electromagnetically coupled to the rotor itself.
In a butterfly valve, there is also present a catch element, which limits the rotation of the rotational shaft forming a mechanical end stroke which defines the maximum closing position reachable by the rotational shaft (i.e. by the butterfly plate). The function of the catch element is to mechanically prevent the butterfly plate from jamming by interference against the feeding pipe, which situation could cause the deformation of the butterfly plate, the deformation of the feeding pipe or, in worse case, the sticking of the butterfly valve.
Currently, the catch element is defined by a catch screw, which is screwed through the valve body and has a head arranged outside the valve body and a free end which defines the mechanical end stroke of the rotational shaft (i.e. of the butterfly plate). During the step of manufacturing, each butterfly valve is arranged in a test station, in which the value of the air flow which flows through the feeding pipe is measured in real time; in these conditions, the axial position of the catch screw is adjusted by screwing or unscrewing the catch screw itself with respect to the valve body, so that when the rotational shaft rests against the catch screw the air flow rate which flows through the feeding pipe is lower than a threshold value established by the design specifications of the butterfly valve. Preferably, after adjusting the axial position of the catch screw, the catch screw itself is locked with respect to the valve body to prevent any type of later movement (typically by effect of the vibrations generated by the engine in use).
After establishing the position of the catch screw, the position sensor is calibrated by defining an offset point corresponding to the position of the rotational shaft resting against the catch screw and then by defining a position sensor gain; subsequently, the software linearization of the position sensor output is performed by using the previously defined offset point and gain.
During the use of the internal combustion engine, the butterfly valve control works to prevent the rotational shaft from coming into contact with the catch screw (except in a highly controlled manner in particular situations and with very slow impact speed); indeed, when the rotational shaft impacts against the catch screw, the gear transmission which transmits the motion from the electric motor to the rotational shaft is subjected to high mechanical stresses which may determine the breakage of the teeth of the gear transmission.
During the use of the internal combustion engine, a self-learning operation is periodically run (typically each time the internal combustion engine is stopped, i.e. in after-run mode) which consists in making the rotational shaft (i.e. the butterfly plate) abut against the catch screw to acquire the offset point again. Such a periodical acquisition of the offset point is necessary because the butterfly valve may get soiled in time and thus an impact which subjects the gear transmission to high mechanical stresses may occur even before the offset point acquired at the end of the manufacturing of the butterfly valve.
From the above, it is apparent that in a known butterfly valve the management of the catch screw is difficult and thus expensive due to the need to calibrate the catch screw and to the need to periodically run a self-learning operation during the use of the internal combustion engine which consists in making the rotational shaft (i.e. the butterfly plate) abut against the catch screw in order to acquire the offset point again.